"The bottom line is that there appears to be a negative impact of highway safety in legalized states, and states considering legalization need to be prepared to deal with this impact", he said.
"The likelihood of being involved in a collision in states with legalized marijuana is higher than neighboring states we used as controls", said David Harkey, IIHS-HLDS president.
In another study researchers compared the number of police-reported crashes from 2012 to 2016 before and after weed hit retail shelves in Colorado, Oregon, and Washington and found a combined 5.2 per cent increase in the rate of crashes per million vehicle registrations, compared to neighboring states that haven't legalized marijuana sales.
Vehicle crashes have risen in states that have legalized marijuana, according to new reports.
"Despite the difficulty of isolating the specific effects of marijuana impairment on crash risk, the evidence is growing that legalizing its use increases crashes", Harkey says. Sales began in October 2015 in OR and in July 2017 in Nevada. On Oct. 17, 2018, Canada will become the second and largest country with a legal national marijuana marketplace.
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Analysts controlled for differences in the rated driver population, insured vehicle fleet, the mix of urban versus rural exposure, unemployment, weather, and seasonality, the institute said. The findings are being presented at a law enforcement summit organized by the two groups.
A positive test for THC [the psychoactive component of marijuana] may not necessarily mean the driver was impaired at the time of the crash.
Other states where Marijuana is either decriminalized or allowed for medical use were not included in this study.
"It's very challenging - we know very well how to measure blood alcohol concentrations, we know how to take those numbers and turn them into laws, such as the.08 laws that we have for legal limits with regards to alcohol, we're not at the same level of knowledge with respect to marijuana", Harkey said. Policies and procedures for drug testing tend to be inconsistent, and many states don't include consistent information on driver drug use in crash reports.
Recreational use isn't allowed in Arizona, but in 2010 voters approved a proposition making marijuana legal for patients with medical cards.